The present invention relates to a record medium reproducing device wherein data from a data storage medium, such as an optical disk, is read and reproduced in response to a command (data read instruction) from a host computer and then delivered to the host computer.
A CD-ROM disk reproducing device is one example of such a storage medium reproducing device. In a CD-ROM disk reproducing device, the accessing of read data from a disk, serving as a data storage medium, and the delivering of the read data to a host computer is carried out as follows:
(a) First, a data read instruction is sent from the host computer to the CD-ROM disk reproducing device, specifying an address of a storage location for data to be read out from the disk.
(b) In the CD-ROM disk reproducing device, when the data read instruction is received, a pickup head for reading and reproducing the data from the disk is moved, as a part of a SEEK operation, to in address position specified by the data read information sent from the host computer.
(c) When data is read from the specified address position by the pickup head, the data is first written in a storage section (buffer RAM) temporarily.
(d) Next, the data from the specified address position written in the buffer RAM is read out and transferred to the host computer.
The accessing of read data obtained from the disk and the delivering of the read data to the host computer is carried out as described above. However, when the CD-ROM disk reproducing device receives a read instruction from the host computer, if data at the address position specified by the instruction has already been written in the buffer RAM and stored therein temporarily, the data need not be read again from the disk, and so moving the pickup head to the specified address position on the disk as part of a SEEK operation is unnecessary. Rather, the data which is already stored temporarily in the buffer RAM may be read out from the buffer RAM directly and transferred to the host computer, with the result that the period of time from the receipt of the data read instruction to the transfer of the requested data to the host computer can be reduced significantly.
The buffer RAM typically used in a CD-ROM disk reproducing device is a high speed, small capacity storage device, also referred to as a cache memory. Because such a memory device has a small capacity, once all storage locations are occupied, new data can not be stored without making room in the buffer RAM by erasing data in selected storage locations, and this is done by using a least-recently-used algorithm, for example. However, once data is erased from the buffer RAM, access to that data can be had only by a time consuming reference to the disk.
Therefore, as indicated in the SCSI-2 (Small Computer System Interface), revision 10C (communication standard regarding computers), among the read data already written in the buffer RAM, for data which is not to be erased by overwriting, but is to be retained for subsequent accesses, the address of the data on the disk specified from the host computer is locked in the buffer RAM by a command from the host computer, thereby providing a technique, known as a Lock Cache function, where data in the buffer RAM, is locked so that it can not be erased, such as by overwriting.
According to the Lock Cache function, data associated with an address position previously accessed from a host computer is always stored in a buffer RAM and may be locked by command from the host computer. If a specified address accompanying a data read instruction from the host computer is coincident with an address position previously accessed from the host computer, the pickup head need not be moved to the assigned address position on the disk, since the data at the accessed address position can be read out directly from the buffer RAM and be transferred immediately to the host computer, if the data is present in the buffer RAM, thereby reducing the time required for data transfer to the host computer following receipt of the data read instruction.
In a record medium reproducing device, such as a CD-ROM disk reproducing device, when such a Lock Cache function is not adopted, or when the Lock Cache function is adopted, but a request for data at an address other than an address previously accessed from the host computer occurs frequently, the SEEK operation involving moving of the pickup head to a specified address position on the disk is necessary. In such case, a problem occurs in that the period of time required from the receipt of the read instruction from the host computer to the time of actual data transfer to the host computer becomes long.
On the other hand, while use of the Lock Cache function serves to retain selected data in the buffer RAM for easy access, the need to control cache locking from the host computer represents an increased overhead which places an undesirable burden on the host computer.